There were a couple of threads last month in this same forum about diving in the DR if you look back. The diving in the DR is in 2 different oceans & they are radically different. On the east & north side of the island you will be diving in the Atlantic, on the south side you will be diving in the Caribbean. The Caribbean waters are clearer, calmer & warmer & the reefs in the Parc Nacional del Este are outstanding. The DR was one of the first countries to make its coral reefs protected areas & the diving there is superb and uncrowded, it seems not many divers know of it. However, it's also not convenient if you are staying in Sosua as it's a long drive. The diving is not great in neither Sosua nor Punta Cana, you are on the Atlantic, & you'll need to take a shuttle bus or cab, the good news is that it used to be a 1 1/2 hour drive to get to Bayahibe or La Romana from Punta Cana, but they have recently opened a new highway & now its 30-45 minutes! Not sure about from Sosua, I don't know if the new highway goes up that far. In the Caribbean you will be fine in a 3 mil wetsuit, we were there at the end of Nov, in the Atlantic its much colder with lots of seaweed.
The last time we were there a couple of years ago we dove with Scubafun out of Bayahibe on the Caribbean, it's a small village close to a larger town called La Romana. That area is considered by most to be the best diving in the DR, there are over 20 dive sites within a short boat ride, 3 wrecks, cave diving, sharks, manatees etc etc etc. The wall diving and reefs out at Catalina Island & Saona are stunning and the dive off Shark Point where the Atlantic Ocean & Caribbean Sea meet is an amazing experience, the line between the 2 bodies of water is sharply defined, both in the water & on the sea bed. They also do night dives with sharks that are very cool. There are a number of dive shops in Bayahibe & all of them will help with divers coming from Punta Cana. Scubafun is owned by a great guy from Arizona, can't remember his name, they gave great service, only 5 of us in the boat with 2 different DM's, both of them excellent. Their website www.scubafun.info has lots of great info about the area including a dive site map & descriptions of the sites.
Unfortunately the summary of all the threads is that by far the best diving in the DR is on the Caribbean side, not on the Atlantic. If the focus of your trip is diving you may want to consider changing your destination or trying to arrange for a couple of day trips to La Romana or Bayahibe, but it will be a long day to travel from Sosua. If you dive the Caribbean side you will have a great time, we think that the DR is one of the most underrated areas in the world to dive.